In many commercial applications, it is necessary to attach a bonded nonwoven material to some other substrate. For example, in baby diapers the inner liner of the diaper is typically a nonwoven fabric comprising rayon, polyester or polypropylene fibers bonded together with some polymeric composition. The outer layer of the diaper is generally a polyethylene film and between the two layers is the absorbent padding.
It is quite apparent from an economic viewpoint that a fast, efficient way to attach the nonwoven inner liner to the polyethylene film is desired by the industry. Additionally, since the inner liner is exposed to urine, it must exhibit good wet strength in order to maintain its integrity and hold the absorbent padding in place. Currently, there is no fast, efficient way of doing this.
The state of the art binder compositions which can produce the necessary wet strength normally have heat seal temperatures that are too high to make such a single step process possible due to prohibitive time and energy costs and the physical melting constraint of the polyethylene film.
This problem exists in the manufacture of diapers, sanitary napkins, and the like and is indicative of similar problems in air-laid type paper embossing.
One approach which is used to solve this problem in the diaper area is a two-step procedure. First the nonwoven inner liner is bonded with a self-crosslinking binder to achieve the necessary wet strength. Then this inner liner is attached to the polyethylene outer layer with an adhesive composition. This procedure is very slow, costly and inefficient.
In air-laid type paper embossing two separate binders may be used. The first binder is a weak wet strength, good heat sealing binder which is used to treat the paper prior to embossing. After embossing, the paper is then treated with a self-crosslinking binder to achieve wet strength. This process also is a slow, inefficient operation which is costly, produces stiff feel to the paper and has tendencies to delaminate binder layers giving weak or inconsistent wet strength.